A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that radiation with a wavelength of less than 10 nm could be used, for example 6.7 nm or 6.8 nm. In the context of lithography, wavelengths of less than 10 nm are sometimes referred to as ‘beyond EUV’.
Extreme ultraviolet radiation and beyond EUV radiation may be produced using a plasma. The plasma may be created for example by directing a laser at particles of a suitable material (e.g. tin), or by directing a laser at a stream of a suitable gas (e.g. Sn vapor, SnH4, or a mixture of Sn vapor and any gas with a small nuclear charge (for example from H2 up to Ar)). The resulting plasma emits extreme ultraviolet radiation (or beyond EUV radiation), which may be collected and focused to a focal point using a collector mirror.
In addition to extreme ultraviolet radiation (or beyond EUV radiation), the plasma produces debris in the form of particles, such as thermalized atoms, ions, nanoclusters, and/or microparticles. The debris may cause damage to the collector mirror (or other components). A buffer gas may be provided in the vicinity of the plasma. The particles produced by the plasma collide with molecules of the buffer gas, and thereby lose energy. In this way, at least some of the particles may be slowed sufficiently that they do not reach the collector mirror. Damage caused to the collector mirror may thereby be reduced. However, even when buffer gas is used, some particles may still reach the collector mirror and cause damage to it.
It is desirable to improve the effectiveness of the buffer gas.